1. Field of the Invention
The present invention relates to a method of adjusting the alignment positions of ink dots printed with at least one printhead that is mounted on a moving carriage of an ink jet printer. The method includes the steps of printing ink dots on a testchart while the carriage moves over the testchart with a predetermined speed, measuring a relative dislocation of the ink dots, and correcting the alignment of the ink dots by adjusting the timing of activation and/or the position of the printhead in accordance with the measured result.
2. Description of Background Art
An ink jet printer typically has one or more printheads mounted on a carriage that is moved over a recording medium in a main scanning direction Y. Thus an image swath consisting of a certain number of pixel lines, corresponding to the number of nozzles of the printhead, is printed during each pass of the carriage. Adjoining swaths of the image are printed in subsequent passes of the carriage, while the recording medium is intermittently advanced in a sub-scanning direction X normal to the main scanning direction Y. In order to obtain a good image quality at the transition from one swath to the other, the ink dots that are printed in different passes have to be aligned correctly in the sub-scanning direction.
However, when an ink droplet is expelled from a nozzle of a printhead, it has to travel a certain distance until it impinges on the recording medium. Since the printhead is moving in the main scanning direction, the ink droplet undergoes a certain speed-dependent aberration in that direction. This may lead to an alignment error between two ink dots that are printed in different passes. For example, when the printer is to be operated in a bi-directional print mode, i.e., a mode in which ink dots are printed in a forward pass and a return pass of the carriage, the aberration depends on the direction of travel of the carriage. The activation timings of the printhead, and hence the positions at which the pertinent nozzles are fired, must therefore be adjusted carefully, so that the different aberrations in the forward pass and the return pass are compensated for.
If the image is printed with a plurality of printheads mounted on the same carriage, the timings and the positions of the printheads on the carriage must be adjusted in order to make sure that the ink dots printed with different printheads have the correct positions relative to one another.
A high quality multi-color printer is preferably equipped with at least two printheads per color. The printheads for the different colors are arranged mirror-symmetrically. One set of color printheads is used only during the forward pass, and the other set of color printheads is used only during the return pass. This has the advantage that the ink dots of different colors will always be superposed in the same sequence, irrespective of the direction of travel of the carriage, so that the color composition will always be the same. However, if the printheads for the same color are not aligned correctly, the ink dots printed with these printheads in the forward and return passes of the carriage will be dislocated relative to one another, so that a thin line extending in the sub-scanning direction X will look rugged.
In a conventional method for checking and adjusting the alignment of the ink dots, the printer is used for printing a test pattern onto a testchart. In that print process, the operating conditions and parameters of the printer are the same as in a normal print operation. In particular, in view of the aberration effect, it is important that the test pattern be printed with a well-defined speed of the carriage. The test pattern on the testchart can then be inspected visually, e.g. with a microscope, or the positions of the ink dots on the testchart may be measured with an electro-optical sensor, in order to provide the data that are needed for correcting the activation timings and/or the printhead positions, if necessary.
A difficulty encountered in detecting the alignment of the ink dots is caused by the fact that, when a nozzle of an ink jet printhead is fired, it normally does not just expel a single droplet, but it first expels a relatively large droplet which is followed by one or more smaller droplets, the so-called satellites. Since the aberration of the satellites is different from that of the main droplet, the corresponding dots formed on the recording medium or the testchart are shifted relative to one another in the main scanning direction, which makes it difficult to detect the exact position of the dot.